Cold-rolled steel plate and method of manufacturing the same

ABSTRACT

The cold-rolled steel plate has a chemical composition containing, on the basis of percent by mass, C from 0.03 to 0.08%, Si from 0 to 1.0%, Mn from 0.2 to 0.8%, P at 0.03% or less, S at 0.01% or less, and Al at 0.05% or less so as to satisfy the following relationship: 5*C %−Si %+Mn %−1.5*Al %&lt;1. The chemical composition further contains, on the basis of percent by mass, at least one of Nb from 0.03 to 0.4%, V from 0.01 to 0.3%, and Ti from 0.01 to 0.3% so as to satisfy the following relationship: 0.04&lt;(Nb %/1.4)+(V %/1.1)+Ti %&lt;0.3. An average diameter of particles of a carbide as a precipitate is from 20 to 100 nm. A second phase structure having a longitudinal diameter of 5 μm or more has an area fraction of 5% or less in a cross-sectional structure.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is the U.S. National Phase of InternationalPatent Application Serial No. PCT/JP2014/051354, filed Jan. 23, 2014,which claims priority to Japanese Patent Application No. 2013-016754,filed Jan. 31, 2013, and Japanese Patent Application No. 2013-222745Oct. 25, 2013. The contents of the foregoing applications are herebyincorporated by reference in their entireties.

TECHNICAL FIELD

This invention relates to a cold-rolled steel plate used for example asa clutch plate, and a method of manufacturing the same.

BACKGROUND ART

A multiplate wet clutch for an automatic transmission has an alternatearrangement of multiple friction plates each with a friction materialmade of a special sheet of paper attached to a surface thereof andseparator plates to contact the friction plates. Action of making aswitch between separating and connecting the friction plates and theseparator plates controls transmission of power.

The friction plate and the separator plate are both ring-shaped steelplate members. Generally, such a friction plate and a separator plateforming the multiplate wet clutch are collectively called a clutchplate.

The following four phenomena are known as major defect phenomenaoccurring in the separator plate: wear of a spline part (hereinaftercalled feature A); a rattle due to an inaccurate position of the splinepart (hereinafter called feature B); change in roughness due to wear ofa surface in friction with the friction plate (hereinafter calledfeature C); and the occurrence of a heat spot and nonuniformity of ashape and a material quality due to the heat spot (hereinafter calledfeature D). All of these phenomena are significant characteristics,among which a defect due to the heat spot is the hardest to deal with.

In response to behavior of the multiplate wet clutch to make a shiftfrom a neutral state to a power transmission state to engage the clutch,the friction plate and the separator plate are pressed against eachother under high load and a high relative velocity. This rapidly reducesthe relative velocity between the friction plate and the separatorplate. Resultant frictional heat rapidly enters a surface of theseparator plate to become a sliding portion, thereby increasing thetemperature of the surface of the separator plate. This temperatureincrease in the surface of the separator plate becomes a cause for theoccurrence of a heat spot.

A projection at the heat spot part resulting from heating with thefrictional heat, distortion occurring around the heat spot, and localchange in a material quality cause a nonuniform frictional state whenthe clutch is actuated. The nonuniform frictional state causes a newheat spot. Such a vicious cycle degrades the performance of themultiplate wet clutch to a large extent.

Enhancing fuel efficiency of automobiles is an extremely importantproblem to be solved at the present time. Enhancing efficiency in termsof a mechanistic aspect and reducing the size and weight of the clutchas a unit are very important elements among various elements forming anautomobile.

Requirements for a transmission include enhancement of efficiency,reduction in friction loss, and reduction in size and weight. Theefficiency of the multiplate wet clutch should be enhanced to satisfythese requirements. The efficiency of the multiplate wet clutch may beenhanced for example by reduction in the diameter of a plate, reductionin the number of plates, and increase in a coefficient of frictionachieved by reducing a lubricant and changing a friction material.However, all of these become causes for excessive temperature increase,specifically, a heat spot that cannot be handled with a conventionaltechnique.

Enhancing the performance of the clutch plate in terms of its materialmay lead to dramatic enhancement of the efficiency of the transmission.Thus, enhancing heat spot resistance is required for a steel plate to beused as the clutch plate.

Methods described, for example, in PTLs 1 to 5 are known as techniquesrelating to enhancement of the heat spot resistance of a steel plate.

According to the method described in PTL 1, temperature of phasetransformation from ferrite to austenite is increased using low-carbonsteel. This prevents the occurrence of phase transformation even if aplate is heated by frictional heat during engagement of a clutch,thereby suppressing the occurrence of a heat spot.

According to the method described in PTL 2, the thermal diffusivity of asteel plate is increased by defining an alloy element content. Thissuppresses temperature increase of a plate to be caused by frictionalheat to suppress the occurrence of a heat spot.

According to the method described in PTL 3, austenitic stainless steelunlikely to be phase transformed is used as a material for a plate,thereby suppressing the occurrence of a heat spot.

According to the method described in PTL 4, a Ti precipitate or an Nbprecipitate is used to suppress the occurrence of a heat spot.

According to the method described in PTL 5, in addition to using a Tiprecipitate or an Nb precipitate, Si or Al having the effect ofincreasing a transformation point is added to suppress the occurrence ofa heat spot.

In addition to enhancement of efficiency, reduction in friction loss,and reduction in size and weight of a transmission achieved by enhancingheat spot resistance, the antiwear performance of a spline part such asa tooth tip of the separator plate is also one important characteristic.

Methods described in PTLs 6 to 9 are known as techniques relating toenhancement of the antiwear performance of a tooth tip of the separatorplate.

According to the method described in PTL 6, antiwear performance isenhanced by using a hard precipitate such as TiC or cementite.

According to the method described in PTL 7, a hot-rolled steel platehaving a ferrite structure with ferrite particles of a diameter of 5 μmor more and 15 μm or less is cold rolled at rolling reduction of 50% ormore, thereby enhancing antiwear performance.

According to the method described in PTL 8, a steel structure iscontrolled through combined addition of Cr, Ti, and B, thereby enhancingantiwear performance.

According to the method described in PTL 9, a steel structure iscontrolled by controlling a fraction of pearlite and that of cementiteand controlling the diameter of ferrite particles, thereby enhancingantiwear performance.

CITATION LIST Patent Literature

PTL 1: Japanese Laid-open Patent Publication No. 2005-249050

PTL 2: Japanese Laid-open Patent Publication No. 2005-249051

PTL 3: Japanese Laid-open Patent Publication No. 2005-249106

PTL 4: Japanese Laid-open Patent Publication No. 2008-266731

PTL 5: Japanese Laid-open Patent Publication No. 2010-132983

PTL 6: Japanese Laid-open Patent Publication No. 2001-73073

PTL 7: Japanese Laid-open Patent Publication No. 2003-277883

PTL 8: Japanese Laid-open Patent Publication No. 2007-211260

PTL 9: Japanese Laid-open Patent Publication No. 2004-162153

SUMMARY OF INVENTION Technical Problem

However, the aforementioned methods of PTLs 1 to 5 are only responsiveto some of the aforementioned four features A to D required to behandled regarding the separator plate. Further, the aforementionedmethods of PTLs 1 to 5 have many problems to be solved including failingto achieve sufficient effect in terms of heat spot resistance, reductionin manufacturing performance, and increase in material cost.

As an example, PTLs 1, 2, and 3 do not consider how to handle thefeatures A, B, and C, failing to achieve sufficient result in terms ofenhancement of efficiency, reduction in friction loss, and reduction insize and weight of a transmission.

Regarding the austenitic stainless steel described in PTL 3, not only isit much more expensive than a steel plate generally used as a clutchplate, but stainless steel also has low heat conductivity which maycause a problem of increasing the temperature of a steel plate surfaceeasily due to low diffusion performance of frictional heat.

The steel compositions described in PTLs 4 and 5 were actually examinedand found to be able to enhance heat spot resistance. However, they failto achieve sufficient results in terms of enhancement of efficiency,reduction in friction loss, and reduction in size and weight of atransmission.

PTL 5 describes addition of Si and Al. However, adding Si and Al causesmany problems in terms of manufacturing and is not effective from anindustrial viewpoint as it is likely to cause brittle fracture of a slabor a coil.

PTLs 6 to 9 are to merely enhance antiwear performance. Specifically,the separator plate is required not only to achieve high antiwearperformance at a spline part but also to avoid damage on a counterpartdrum or a counterpart case. Thus, simply being capable of enhancingantiwear performance is not sufficient for a material for the separatorplate.

Controlling change in roughness of a surface in friction with thefriction plate is also a different significant characteristic relatingto wear or a wear phenomenon. Specifically, the performance of a surfaceof the separator plate against wear caused by a friction sheet of paperas a counterpart material is an important issue. Damage on the frictionsheet of paper as a counterpart material of friction should certainly beavoided.

None of PTLs 6 to 9 gives consideration to providing both of two typesof antiwear performances of different features at two sites, a splinepart and a surface.

Regarding the property of a steel plate, bad punching performance lowersa shear surface ratio during punching into a plate of a given shape.This makes the occurrence of a flash or a burr and a secondary shearsurface likely.

The nature of a punching surface degraded in this way becomes a causefor rattle, wear, damage or the like of a spline part if the steel plateis used as a clutch plate, for example.

Thus, a steel plate with favorable punching performance has beenrequired as a material for a clutch plate, for example.

This invention has been made in view of the aforementioned issues. It isan object of this invention to provide a cold-rolled steel plate withfavorable punching performance and a method of manufacturing the same.

Solution to Problem

A cold-rolled steel plate can have a chemical composition containing, onthe basis of percent by mass, C from 0.03 to 0.08 %, Si from 0 to 1.0 %,Mn from 0.2 to 0.8%, P at 0.03% or less, S at 0.01% or less, and Al at0.05% or less as to satisfy a formula (1): 5*C %−Si %+Mn %−1.5*Al %<1,and at least one of Nb from 0.03 to 0.4%, V from 0.01 to 0.3%, and Tifrom 0.01 to 0.3% so as to satisfy a formula (2): 0.04<(Nb %/1.4)+(V%/1.1)+Ti %<0.3, with a residue being formed of Fe and unavoidableimpurities. An average diameter of particles of a carbide as aprecipitate containing any of Nb, V and Ti is from 20 to 100 nm. Asecond phase structure having a longitudinal diameter of 5 μm or morehas an area fraction of 5% or less in a cross-sectional structure. Thecold-rolled steel plate has cross-sectional hardness from 200 to 350 HV.

According to a cold-rolled steel plate can have a chemical compositionthat contains, on the basis of percent by mass, at least one of Cr from0.10 to 2.0%. Ni from 0.05 to 0.5%, Mo from 0.05 to 0.5%, and B from0.0002 to 0.002% so as to satisfy a formula (3): 5*C %−Si %+Mn %+1.6*Cr%+0.8*Ni %−1.5*Al %<1.

The average diameter of particles of the carbide containing any of Nb,V, and Ti is from 20 to 100 nm. The carbide is a precipitate in a frontlayer part extending at least 200 μm from a surface of the steel plate.

According to a method of manufacturing a cold-rolled steel plate, asteel slab having the chemical composition as recited above is smelted,the steel slab is heated to 1200° C. or more and hot rolled to form ahot-rolled steel plate, the hot-rolled steel plate is wound from 550 to700° C. to form a hot-rolled coil, and the hot-rolled coil is coldrolled or the hot-rolled coil is annealed and cold rolled, therebyobtaining cross-sectional hardness from 200 to 350 HV.

The cold-rolled steel plate can have an average diameter of particles ofthe carbide containing any of Nb, V, and Ti is from 20 to 100 nm. Thecarbide is a precipitate in a front layer part extending at least 200 μmfrom a surface of the steel plate.

According to a method of manufacturing a cold-rolled steel plate thesteel slab is heated to 1200° C. or more and hot rolled to form ahot-rolled steel plate, the hot-rolled steel plate is wound from 550 to700° C. to form a hot-rolled coil, and the hot-rolled coil is coldrolled or the hot-rolled coil is annealed and cold rolled, therebyobtaining cross-sectional hardness from 200 to 350 HV.

Advantageous Effects of Invention

According to this invention, the chemical composition is regulated, theaverage diameter of particles of the precipitated carbide is from 20 to100 nm, and the second phase structure having a longitudinal diameter of5 μm or more has an area fraction of 5% or less in the cross-sectionalstructure, thereby being able to enhance punching performance. Thecold-rolled steel plate of this invention is used appropriately not onlyas a separator plate but also as a friction plate or a dry clutch plate.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a perspective view showing a state before a test to examineheat spot resistance of a Working Example of this invention isconducted.

FIG. 2A is a plan view showing a state after the test to examine theheat spot resistance is conducted, and FIG. 2B is a sectional view takenalong A-A of FIG. 2A showing the state after the test to examine theheat spot resistance is conducted, and FIG. 2C shows a site wherehardness is measured.

DESCRIPTION OF EMBODIMENTS

The following describes an embodiment of this invention.

A cold-rolled steel plate of this invention is used for example as amaterial and the like for a clutch plate in a multiplate wet clutchmechanism of an automatic transmission of an automobile.

A cause for the occurrence of a heat spot in a clutch plate made of anormal steel plate is described first.

If a clutch is engaged under high load, temperature increasesconsiderably by friction in a surface of the clutch plate in the engagedstate. This austenitizes a metal structure of the steel plate. In thesteel plate, a region where austenitization occurs in response totemperature increase during engagement of the clutch is limited to afront layer of the steel plate. Temperature increase to a degree thatcauses phase transformation does not occur inside the steel plate.

The heated region in the surface of the steel plate is rapidly cooled(self-cooled) resulting from rapid heat conduction to the inside of thesteel plate in a low temperature to be martensitically transformed.

If the part heated by friction is rapidly cooled by self-cooling to bemartensitically transformed, a resultant martensitic structure expandsin volume to create a heat spot that is a region like a projectionprojecting to a higher position than its surrounding area.

The change in shape occurring during the martensitic transformationapplies compressive residual stress to a surrounding structure. Thisdamages the flatness of the clutch plate to distort the clutch plate.

Such a heat spot is suppressed effectively by the following first tofourth countermeasures.

The first countermeasure is to suppress temperature increase of theclutch plate caused by frictional heat. More specifically, the heatconductivity of the steel plate forming the clutch plate is increased.This rapidly diffuses heat from a friction part in the surface of theclutch plate to a surrounding area, thereby suppressing abnormaltemperature increase to occur locally in the outermost surface.

The heat conductivity of steel becomes highest in pure iron and becomeslower with increase in an alloy element added content. The heatconductivity also becomes lower with increase in the volume fraction ofa second phase such as pearlite. Meanwhile, adding an alloy element intosteel is necessary for ensuring appropriate strength and antiwearperformance of the steel plate as a clutch plate.

Strength and antiwear performance required for the steel plate as aclutch plate are ensured and high heat conductivity is maintainedeffectively by forming a dispersed metal structure with fineprecipitates dispersed uniformly in a ferrite structure.

The second countermeasure is to suppress transformation into the γ phaseof a metal structure in a front layer part of the clutch plate even ifthe temperature of the clutch plate is increased by frictional heat.More specifically, even if temperature increase of the surface of thesteel plate by the frictional heat is unavoidable, austenitization tooccur resulting from temperature increase by friction is stillsuppressed by making austenite transformation of the steel plate itselfunlikely.

Austenite transformation is suppressed effectively by increasing atransformation point or delaying dissolving of a carbide.

A transformation point is increased effectively by adding an element toincrease a point of α to γ transformation (transformation point A₃) orreducing the added content of an element to reduce the transformationpoint A₃.

Dissolving is delayed effectively by making a carbide exist in steelwith stable properties that prevent dissolving of the carbide as much aspossible.

In carbon steel, α to γ transformation starts with dissolving of acarbide at an interface between the carbide and the mother phase. If thecarbide is dissolved in the α phase or the γ phase easily, the α to γtransformation proceeds promptly. Meanwhile, if the carbide is notdissolved easily, progress of the α to γ transformation is suppressed.

The transformation point drops with C, Mn, and Ni, and increases with Siand Cr. Thus, it is important to minimize the respective added contentsof C, Mn, and Ni as much as possible. Where needed, the respective addedcontents of Si and Cr may be increased in consideration of otherconditions, for example.

An Fe₃C (cementite: θ)-based material as a carbide has the property ofbeing dissolved easily. Meanwhile, in the case of steel containing Cr,Cr tends to be concentrated in Fe₃C. The concentration of Cr stabilizesFe₃C. Compared to Fe₃C, an Nb-based carbide, a V-based carbide, or aTi-based carbide has the property of being remarkably stable and havinga low degree of solubility in the γ phase.

Specifically, dispersing an Nb-based carbide, a V-based carbide, or aTi-based carbide finely and uniformly is a considerably excellent methodof ensuring the strength and antiwear performance of the steel plate.Thus, using an Nb-based carbide, a V-based carbide, or a Ti-basedcarbide is a preferable method of suppressing transformation into the γphase. Adding Nb, V, or Ti is also an effective method of reducing C asan element that reduces a transformation point most prominently.

Therefore, strength and antiwear performance are ensured effectively byadding Nb, V, or Ti and forming a hard carbide. With the intention ofreducing the amount of redundant C not to be bonded to Nb, V, or Ti, anoptimum added content is determined in consideration of a relationshipof an Nb added content, a V added content, or a Ti added content withthe amount of C. This reduces the amount of dissolved C in a part heatedby friction, thereby being able to suppress transformation into the γphase.

The third countermeasure is to suppress martensitic transformation to becaused by self-cooling of the clutch plate even if temperature increaseby frictional heat transforms a metal structure in a front layer part ofthe clutch plate into the γ phase. More specifically, even iftemperature increase and transformation into the γ phase of the surfaceof the steel plate are unavoidable, martensitic transformation to becaused by self-cooling is still suppressed by reducing the quenchingperformance of the steel plate.

The quenching performance is reduced effectively by reducing the addedcontent of an element to enhance the quenching performance and by makingthe diameter of γ crystal grains finer.

What is important is to minimize the respective added contents of Si,Mn, Ni, Cr, Mo, B, etc. to reduce (or to not enhance) the quenchingperformance.

The diameter of γ crystal grains is made finer effectively through useof a grain boundary pinning effect achieved by a fine precipitate. Morespecifically, an Nb-based carbide, a V-based carbide, a Ti-basedcarbide, and a nitride are dispersed finely to make the diameter of γgrains finer. This facilitates nucleation in the α phase during coolingfrom the γ phase, thereby reducing the quenching performance. Such useof Nb, V, and Ti is considerably effective as it not only acts toenhance heat conductivity and suppress transformation into the γ phasebut also acts to enhance strength and antiwear performance.

The fourth countermeasure is to suppress deformation of the clutch platedue to transformation stress even if temperature increase by frictionalheat transforms a metal structure in the front layer part of the clutchplate into the γ phase to martensitically transform the metal structureas a result of self-cooling. More specifically, even if martensitictransformation (conversion to a heat spot) of a heated part in thesurface of the steel plate is unavoidable, distortion of the clutchplate due to the heat spot is suppressed by ensuring sufficient strengthof a metal structure surrounding the heat spot.

As described above, the heat spot itself is considered to be amartensitic region resulting from heating and rapid cooling of afriction part. Though not as high as in the heat spot, temperatureincreases in a surrounding area of the heat spot resulting fromfrictional heat to affect the metal structure. The heat spot issubjected to phase transformation from α to γ and then to martensite.However, the surrounding of the heat spot is not heated to a degree thatcauses transformation into the γ phase, so that it becomes soft in manycases resulting from its material structure. More specifically, a steelplate generally used as a clutch plate has hardness controlled to befrom about 220 to about 320 HV through hardening by cold rolling. In thesurrounding area of the heat spot, the cold rolling causes recovery andrecrystallization of a worked structure with heat resulting fromfrictional heat, thereby reducing hardness.

Thus, softening of the surrounding area of the heat spot can besuppressed by suppressing recovery and recrystallization of acold-rolled structure. More specifically, by adding Nb, V, or Ti andforming a structure containing a thermally stable Nb-based carbide,V-based carbide, or Ti-based carbide dispersed uniformly in thestructure, recrystallization can be suppressed and reduction in hardnesscan be suppressed effectively.

Punching performance is determined based on the nature of a punchingsurface resulting from a general-purpose punching method. An excellentpunching surface has a high shear surface ratio, a low probability of aburr or a flash, and a low probability of a secondary shear surface at across section subjected to a punching process. Specifically, reducingthe probability of a fracture surface, a secondary shear surface, and aburr (flash) at the processed cross section suppresses the occurrence offine powder (contaminants) due to these factors. This makes it possibleto reduce a cause for a defect to occur inside a transmission if thesteel plate is used as a clutch plate, for example.

The punching performance of the steel plate is enhanced effectively bymaking a cross-sectional structure a substantially single phasestructure of ferrite and making a second phase exist uniformly in smallquantities. Suppressing formation of the cross-sectional structure intoa structure with bands is also an effective way. Providing the steelplate with appropriate hardness is another effective way.

Based on the aforementioned first, second, third, fourthcountermeasures, etc. and the aforementioned countermeasures to be takenregarding punching performance, the chemical composition of acold-rolled steel plate is defined as follows. Unless otherwise stated,the content of each element is expressed on the basis of percent bymass.

Specifically, the cold-rolled steel plate has a chemical compositioncontaining C from 0.03 to 0.08%, Si from 0 to 1.0% (including a casewhere Si is not added), Mn from 0.2 to 0.8%, P at 0.03% or less(excluding a case where P is not added), S at 0.01% or less (excluding acase where S is not added), and Al at 0.05% or less (excluding a casewhere Al is not added) so as to satisfy a formula (1): 5*C %−Si %+Mn%−1.5*Al %<1, and at least one of Nb from 0.03 to 0.4%, V from 0.01 to0.3%, and Ti from 0.01 to 0.3% so as to satisfy a formula (2): 0.04<(Nb%/1.4)+(V %/1.1)+Ti %<0.3, with a residue being formed of Fe andunavoidable impurities. In the formula (1), C % means the content of C(%), Si % means the content of Si (%), Mn % means the content of Mn (%),and Al % means the content of Al (%). In the formula (2), Nb % means thecontent of Nb (%), V % means the content of V (%), and Ti % means thecontent of Ti (%).

If the occasion arises, at least one of Cr, Ni, Mo, and B may be addedas follows to the aforementioned chemical composition.

More specifically, in addition to the aforementioned chemicalcomponents, the chemical composition may contain at least one of Cr from0.10 to 2.0%, Ni from 0.05 to 0.5%, Mo from 0.05 to 0.5%, and B from0.0002 to 0.002% so as to satisfy a formula (3): 5*C %−Si %+Mn %+1.6*Cr%+0.8*Ni %−1.5*Al %<1. In the formula (3). C % means the content of C(%), Si % means the content of Si (%), Mn % means the content of Mn (%),Cr % means the content of Cr (%), Ni % means the content of Ni (%), andAl % means the content of Al (%).

Each element and the content of each element in the cold-rolled steelplate are described below.

Regarding C (carbon), a content below 0.03% makes it difficult to formhard carbide particles to contribute to antiwear performance. Meanwhile,in response to increase in the content of C, the point of transformationfrom α to γ drops, hardness and a dilation deformation amount areincreased in a part heated by frictional heat during formation of amartensitic structure, and thermal conductivity is reduced. Further,increase in the content of C generates more hard structures such as apearlite structure, a bainite structure, a cementite phase (Fe₃C), or aferrite structure containing fine cementite dispersed in grains, therebydegrading punching performance. Then, an upper limit of the content of Cdetermined within a range satisfying the relationship of the formula (1)or (3) in consideration of a balance with other characteristics is0.08%. Thus, the content of C is determined to be from 0.03% or more and0.08% or less.

Regarding Si (silicon), a content below 0.4% is sufficient if Si is tobe added for the general purpose of deoxidization. Meanwhile, Si has theeffect of increasing the point of transformation from α to γ, so thatthe content of Si may exceed 0.4%. Adding Si excessively to a contentexceeding 1.0% is likely to cause brittle fracture during rolling of asteel plate, for example. Thus, the content of Si is determined to be(0% (including a case where Si is not added) or more and 1.0% or less.

Regarding Mn (manganese), Mn is an element necessary for enhancing thestrength of a material steel plate and should be added to a content of0.2% or more for enhancing the strength. Meanwhile, Mn has the effect ofreducing the point of transformation from α to γ. Thus, adding Mn to acontent exceeding 0.8% reduces the point of transformation from α to γ.Thus, the content of Mn is determined to be from 0.2% or more and 0.8%or less. As the content of Mn increases, a hot-rolled steel plate ismore likely to be formed into a structure with bands and the nature ofthe hot-rolled steel plate at a punching cross section formed by apunching process is more likely to be degraded. Thus, it is morepreferable that the content of Mn be 0.6% or less.

Regarding P (phosphorus), adding P to a content exceeding 0.03% reducespunching performance and toughness. Thus, the content of P is determinedto be 0.03% or less (excluding a case where P is not added).

Regarding S (sulfur), S forms MnS. Adding S to a content exceeding 0.01%makes the occurrence of a fracture surface likely in a cross-sectionalstructure due to soft MnS expanded through rolling. Thus, the content ofS is determined to be 0.01% or less (excluding a case where S is notadded).

Regarding Al (aluminum), Al is an element having deoxidizing effect. Acontent below 0.01% is sufficient if Al is to be added only for thepurpose of deoxidization. Meanwhile, Al has the effect of increasing thepoint of transformation from α to γ, so that the content of Al mayexceed 0.01%. In the case of steel containing Nb, V, or Ti to a givenconcentration, adding Al in large quantities to this steel to a contentexceeding 0.05% does not work advantageously in terms of the effect ofincreasing a transformation point. Thus, the content of Al is determinedto be 0.05% or less (excluding a case where Al is not added).

Regarding Cr (chromium), Ni (nickel), Mo (molybdenum), and B (boron),these elements have the effect of enhancing antiwear performance andtoughness. Thus, it is preferable that these elements be added if acounterpart spline to make a fit with a spline part of a separator plateis hard as a result of a surface hardening process such as carburizingor nitriding, for example.

If Cr is to be added, the content of Cr is determined to be 0.10% ormore and 2.0% or less in consideration of its effect of enhancingantiwear performance and its side effect.

If Ni is to be added, the content of Ni is determined to be 0.05% ormore and 0.5% or less in consideration of its effect of enhancingtoughness and its side effect.

If Mo is to be added, the content of Mo is determined to be 0.05% ormore and 0.5% or less in consideration of its effect of enhancingtoughness and its side effect.

If B is to be added, the content of B is determined to be 0.0002% ormore and 0.002% or less in consideration of its effect of enhancingtoughness and its side effect.

If Cr or Ni is to be added, the effect of Cr or that of Ni affects thepoint of transformation from α to γ and the quenching performance of acold-rolled steel plate. To increase the point of transformation from αto γ and to reduce the quenching performance, the respective contents ofC, Si, Mn, and Al, and those of Cr and Ni should be consideredcomprehensively so as to satisfy the relationship defined by the formula(3): 5*C %−Si %+Mn %+1.6*Cr %+0.8*Ni %−1.5*Al %<1.

Regarding Nb (niobium), V (vanadium), and Ti (titanium), Nb, V, and Tiare each bonded to C in steel to form a hard carbide, contributing toenhancement of antiwear performance. Further, Nb, V, and Ti have theeffect of fixing carbon in the steel to become NbC, VC, and TiCrespectively of low degrees of solubility and suppressing α to γtransformation in a part heated by frictional heat. Additionally, Nb, V,and Ti effectively suppress coarsening of the diameter of ferritecrystal grains and softening in a part where a temperature is to beincreased by friction. That is, containing Nb, V or Ti contributes toenhancement of heat spot resistance and antiwear performance.

To achieve the aforementioned effect relating to heat spot resistanceand antiwear performance by adding at least one of Nb, V and Ti, Nb, V,and Ti should be added to 0.03% or more, 0.01% or more, and 0.01% ormore respectively. Meanwhile, containing Nb, V, and Ti at a contentexceeding 0.4%, a content exceeding 0.3%, and a content exceeding 0.3%respectively increases the hardness of a hot-rolled steel plate. Thismakes it impossible to manufacture steel for a plate of an intendedthickness and intended hardness of the plate as a product. Thus, thecontent of Nb is determined to be 0.03% or more and 0.4% or less, thatof V is determined to be 0.01% or more and 0.3% or less, and that of Tiis determined to be 0.01% or more and 0.3% or less.

Heat spot resistance, antiwear performance, and other side effects areaffected by the respective effects of the elements Nb, V, and Ti to beadded. Thus, the respective contents of these elements should beconsidered comprehensively. At least one of Nb, V, and Ti should beadded within the aforementioned range of the corresponding contents ofNb, V, and Ti so as to satisfy the relationship defined by the formula(2): 0.04<(Nb %/1.4)+(V %/1.1)+Ti %<0.3.

An Nb-based carbide, a V-based carbide, and a Ti-based carbide areconsiderably significant elements for enhancing heat spot resistance andantiwear performance. Specifically, an Nb-based carbide, a V-basedcarbide, and a Ti-based carbide in a surface of a steel plate exerttheir effects for enhancing heat spot resistance and antiwearperformance at a surface of a spline part in friction with a counterpartspline. For this purpose, the Nb-based carbide, the V-based carbide, andthe Ti-based carbide should be dispersed finely and uniformly.

More specifically, an average diameter of particles of a precipitate ina steel plate, specifically that of particles of a carbide containingany of Nb, V, and Ti should be in a range from 20 nm or more and 100 nmor less.

In particular, an Nb-based carbide, a V-based carbide, and a Ti-basedcarbide existing in a surface of the steel plate and a front layer partnear the surface largely affect heat spot resistance and antiwearperformance. Thus, it is preferable that an average diameter ofparticles of a carbide containing any of Nb, V, and Ti be 20 am or moreand 100 nm or less existing in the front layer part, which is a layerextending at least 200 μm from the surface of the steel plate.

Meanwhile, an Nb-based carbide, a V-based carbide, and a Ti-basedcarbide existing in positions such as those in a central part of thecross-sectional direction of the steel plate deeper than the front layerpart do not contribute much to heat spot resistance. If given excellentantiwear performance, these carbides in turn cause the risk of damage ona counterpart material. Thus, an average diameter of particles of theNb-based carbide, that of particles of the V-based carbide, and that ofparticles of the Ti-based carbide existing in the central part of thecross-sectional direction of the steel plate are only required to besubstantially the same as those of the corresponding particles in thefront layer part. An Nb-based carbide, a V-based carbide, and a Ti-basedcarbide in the central part of the cross-sectional direction existing inexcessively larger quantities than the corresponding carbides in thefront layer part in turn are not preferable when using a cold-rolledsteel plate as a clutch plate. Thus, like that of particles of a carbidecontaining any of Nb, V, and Ti in the front layer part, it ispreferable that an average diameter of particles of a carbide containingany of Nb, V, and Ti in the central part of the cross-sectionaldirection of the steel plate or that of particles of such a carbide in alayer deeper than the depth of 200 μm from the surface of the steelplate be 20 nm or more and 100 nm or less.

In addition to Nb, V, and Ti, W (tungsten), Ta (tantalum), Zr(zirconium), and Hf (hafnium) may be added as elements to form hardcarbides.

A cross-sectional structure includes the ferrite phase as a motherphase, and second phase structures harder than the ferrite phaseincluding a pearlite structure, a bainite structure, a cementitestructure, and a structure of a second phase other than cementitedispersed finely in a ferrite structure. A difference in hardness isgenerated in the cross-sectional structure between these second phasestructures as hard structures and a ferrite base material softer thanthe second phase structures. If the difference in hardness between theferrite base material and the second phase structures becomes largedepending on the quantities, sizes or hardness of the second phasestructures dispersed in the cross-sectional structure, a difference indeformability is generated to make the occurrence of a crack likely atan interface between the second phase structure and the ferritestructure resulting from deformation caused by a punching process. As aresult, a fracture surface is caused easily to reduce a primary shearsurface ratio.

Thus, it is preferable that, for enhancement of punching performance, asteel plate have a cross-sectional structure in a substantially singlephase structure of ferrite containing a second phase existing uniformlyin small quantities. The area fraction of a second phase structure inthe cross-sectional structure is an important issue.

If the area fraction of a second phase structure having a longitudinaldiameter of 5 μm or more exceeds 5% in the cross-sectional structure,this second phase structure exerts prominent influence to reducepunching performance. Thus, the volume fraction of the second phasestructure having a longitudinal diameter of 5 μm or more is determinedto be 5% or less in the cross-sectional structure of a steel plate. Thesize, quantity, or hardness of this second phase structure in thecross-sectional structure can be adjusted, for example, by controllingthe content of C in the steel plate or a temperature of windingperformed after hot rolling. Generally, the size of the second phasestructure is determined based on the longitudinal diameter thereof,which is a length in a direction where this second phase structuregrows.

A manufacturing method of this invention is described next.

First, a steel slab is smelted that has a chemical compositioncontaining C from 0.03 to 0.08%, Si from 0 to 1.0% (including a casewhere Si is not added), Mn from 0.2 to 0.8%, P at 0.03% or less(excluding a case where P is not added), S at 0.01% or less (excluding acase where S is not added), and Al at 0.05% or less (excluding a casewhere Al is not added) so as to satisfy the formula (1), and at leastone of Nb from 0.03 to 0.4%, V from 0.01 to 0.3%, and Ti from 0.01 to0.3% so as to satisfy the formula (2), with a residue being formed of Feand unavoidable impurities.

If at least one of Cr, Ni, Mo, and B is to be added, the chemicalcomposition of the smelted steel slab contains, in addition to each ofthe aforementioned chemical components, at least one of Cr from 0.10 to2.0%, Ni from 0.05 to 0.5%, Mo from 0.05 to 0.5%, and B from 0.0002 to0.002% so as to satisfy the formula (3).

This steel slab is heated to 1200° C. or more and then hot rolled. Aheating temperature below 1200° C. leads to the probability of failingto dissolve a carbide sufficiently.

For the hot rolling, it is preferable that a hot rolling finishingtemperature be set to be higher than an Ar₃ transformation point interms of the quality of a hot-rolled steel plate and hot rollingefficiency, for example. Specifically, it is preferable that thefinishing temperature be determined to be 850° C. or more and 950° C. orless.

A winding temperature for the hot-rolled steel plate below 550° C.produces many hard structures. Determining the winding temperature to be550° C. or more can reduce a hard structure to produce a structure closeto a single phase of ferrite. Meanwhile, a winding temperature exceeding700° C. decarburizes a surface of the steel plate notably. This reducesthe quantity of the precipitate of the carbide in the outermost layerpart and reduces the diameter of particles of the carbide. Thus, thewinding temperature is determined to be 550° C. or more and 700° C. orless and a hot-rolled coil formed by winding the hot-rolled steel platein this temperature range is used as a material.

Moderate cooling to reduce a temperature from the finishing temperatureto the winding temperature at an average cooling rate below 20° C. persecond coarsens the precipitated carbide. Thus, it is preferable thatthe average cooling rate be 20° C. per second or more.

The hot-rolled steel plate is subjected to acid pickling to removescales from the surface and is then cold rolled to become a product.More specifically, to obtain hardness required for the steel plate as aclutch plate, particularly as a separator plate, the steel plate shouldbe cold rolled at a cold rolling ratio of 20% or more and 70% or less.The hardness is controlled by controlling the rolling ratio.

A cold-rolled steel plate used as a product is required to have hardnessof 200 HV or more and 350 HV or less and flatness in terms of punchingperformance. To ensure flatness, it is preferable that the cold rollingratio be controlled in a range of 20% or more and 70% or less. Hardnessbelow 200 HV causes serious sagging and a serious burr of a punched itemand causes a secondary shear surface, degrading the nature of a splinepan. Meanwhile, hardness exceeding 350 HV causes serious wear or damageof a punch die while failing to form a shear surface in a punchingsurface, which is not preferable as the shape of a spline part.

If the occasion arises, annealing may be performed directly on thehot-rolled steel plate or may be performed as intermediate annealing onthe cold-rolled steel plate. In either case, it is preferable that thesteel plate be cold rolled after the annealing. If the intermediateannealing is to be performed during the cold rolling process, a suitableannealing condition can be selected appropriately in view of thethickness of a product and the cold rolling ratio. However, an annealingcondition to cause surface decarburization is not preferable.

Regarding the aforementioned cold-rolled steel plate, the chemicalcomposition is regulated based on the aforementioned first to fourthcountermeasures and the aforementioned countermeasures to be takenregarding punching performance. Further, an average diameter ofparticles of a precipitated Nb-based carbide, that of particles of aprecipitated V-based carbide, or that of particles of a precipitatedTi-based carbide is determined to be 20 nm or more and 100 nm or less.Additionally, a hard structure having hardness of 200 HV or more and alongitudinal diameter of 5 μm or more is determined to occupy a volumefraction of 5% or less in a cross-sectional structure, thereby beingable to enhance punching performance.

According to the conventional techniques, an alloy element added contentshould be reduced to enhance heat spot resistance while an alloy elementrequired for enhancing antiwear performance should be added to enhancestrength. Thus, enhancement of heat spot resistance and that of antiwearperformance cannot be well balanced. Meanwhile, the aforementionedcold-rolled steel plate can enhance heat spot resistance and antiwearperformance in a well-balanced manner. In addition to balancing heatspot resistance and antiwear performance with each other, theaforementioned cold-rolled steel plate achieves enhancement of punchingperformance.

As a result, a punching surface can be provided with excellent naturethrough a conventional punching process. Specifically, the occurrence ofa fracture surface, a secondary shear surface, and a burr or a flash canbe suppressed in the punching surface. This can reduce a cause for adefect to occur inside a transmission if the cold-rolled steel plate isused as a material for a clutch plate, for example.

A fine and hard carbide dispersed in the steel plate achieves the effectof suppressing a plastic flow and a micro fracture in a friction part,thereby being able to suppress adhesive wear resulting from frictionbetween steel and steel.

With the excellent nature of a punching surface, a clearance from acounterpart spline part can be set to be small. This can suppress arattle occurring during actuation, thereby reducing an impact on aspline part. If the punching surface is flat, a contact surface pressureis reduced to suppress wear that is to occur resulting from contact withthe counterpart spline part. This suppresses an impact or a surfacepressure on the counterpart spline part, thereby reducing wear on theside of a material as a contact counterpart.

As seen from the foregoing, the aforementioned cold-rolled steel platehas excellence in terms of antiwear performance at a spline part(suppression of wear of the plate itself) and aggression toward amaterial as a contact counterpart (suppression of wear and damage of acounterpart spline part). From a viewpoint of these aspects relating toantiwear performance, the aforementioned cold-rolled steel plate is usedappropriately as a material for a clutch plate, for example.

The aforementioned cold-rolled steel plate is used appropriately notonly as a separator plate of a multiplate wet clutch but also as afriction plate of the multiplate wet clutch or a dry clutch plate.

EXAMPLES

Working Examples and Comparative Examples are described below.

Steel slabs having chemical compositions shown in Table 1 were smelted.Referring to Table 1, if at least one of Cr, Ni, Mo, and B was notadded, a value of the left side of the formula (1) is shown as a Qvalue. If at least one of Cr, Ni, Mo, and B was added, a value of theleft side of the formula (3) is shown as the Q value.

TABLE 1 (Percent by mass) Steel C Si Mn P S Cr Ti Nb Ni V Mo B Al Qvalue Formula (2) Comparative 1 0.07 0.28 0.54 0.012 0.006 — 0.01 — — —— — 0.010 0.60 0.01 Examples 2 0.22 0.22 0.48 0.016 0.003 — 0.17 0.05 —— — — 0.007 1.35 0.21 3 0.12 0.07 0.67 0.015 0.005 — 0.17 — — — — —0.012 1.18 0.17 5 0.10 1.23 0.44 0.014 0.004 — 0.10 — — — 0.08 — 0.015−0.31 0.10 6 0.02 0.04 0.41 0.009 0.005 — 0.11 — — — — — 0.009 0.46 0.117 0.07 0.06 0.32 0.015 0.005 — 0.47 — — — — — 0.011 0.59 0.47 8 0.110.06 0.37 0.016 0.007 — 0.16 0.35 — — — — 0.011 0.84 0.41 9 0.08 0.051.30 0.011 0.004 — 0.06 0.01 — — — — 0.190 1.37 0.06 10 0.15 0.05 1.320.016 0.004 — 0.22 — — — — — 0.041 1.96 0.22 Working Example 11 0.070.11 0.41 0.011 0.004 — 0.07 — — — — — 0.006 0.64 0.07 ComparativeExample 12 0.11 0.07 0.38 0.009 0.003 — 0.14 — — — — — 0.011 0.84 0.14Working 13 0.07 0.03 0.38 0.014 0.006 — 0.10 0.07 — — — — 0.009 0.690.15 Examples 14 0.05 0.05 0.35 0.008 0.004 0.11 0.25 0.06 — — — — 0.0070.72 0.25 15 0.06 0.07 0.40 0.012 0.003 — 0.09 — — — — — 0.010 0.62 0.0916 0.05 0.12 0.50 0.009 0.004 — 0.10 — — — — 0.001 0.010 0.62 0.14 170.06 0.05 0.36 0.012 0.003 — 0.08 — — 0.10 — — 0.011 0.59 0.17 18 0.050.07 0.39 0.009 0.005 — 0.10 — — — 0.13 — 0.009 0.56 0.10 19 0.05 0.110.41 0.010 0.005 — 0.09 — — 0.11 — — 0.008 0.54 0.19 Comparative Example20 0.11 0.15 0.55 0.013 0.005 — 0.07 — — — — — 0.013 0.93 0.07 WorkingExamples 21 0.07 0.78 0.72 0.014 0.004 — 0.16 — — — — — 0.020 0.26 0.1622 0.07 0.15 0.44 0.011 0.003 — 0.07 — 0.43 — — — 0.020 0.95 0.07 230.04 0.05 0.40 0.012 0.005 — 0.08 — — — — — 0.018 0.52 0.08 24 0.03 0.040.31 0.009 0.004 — 0.05 — — — — 0.002 0.015 0.40 0.05 25 0.04 0.07 0.520.013 0.006 — 0.06 — — 0.05 — — 0.013 0.63 0.11 26 0.04 0.04 0.37 0.0130.005 — — 0.10 — — — — 0.024 0.49 0.07 27 0.05 0.05 0.39 0.011 0.004 — —0.35 — — — — 0.020 0.56 0.25 28 0.05 0.03 0.35 0.010 0.003 — — — — 0.07— — 0.022 0.54 0.06 29 0.06 0.05 0.36 0.014 0.005 — — — — 0.27 — — 0.0140.59 0.25 30 0.04 0.04 0.40 0.015 0.003 — — 0.15 — 0.10 — — 0.028 0.520.20 31 0.06 0.02 0.38 0.012 0.004 — 0.12 0.10 — 0.10 — — 0.020 0.630.28 32 0.05 0.03 0.39 0.010 0.004 — 0.06 0.08 — 0.05 — — 0.022 0.580.16 33 0.05 0.05 0.36 0.011 0.005 — 0.03 0.02 — 0.02 — — 0.019 0.530.06 Comparative Examples 34 0.11 0.05 0.36 0.012 0.003 — — 0.12 — 0.08— — 0.024 0.82 0.16 35 0.10 0.06 0.38 0.014 0.005 — — 0.32 — — — — 0.0210.79 0.23 36 0.04 0.07 0.45 0.009 0.014 — 0.09 — — — — — 0.021 0.55 0.09Working Example 37 0.06 0.21 0.62 0.011 0.008 — 0.11 — — — — — 0.0250.67 0.11

Using various types of steel slabs, hot rolling and cold rolling wereperformed under conditions shown in Table 2 to produce various types ofsamples.

The hot rolling was performed at a heating temperature of 1250° C. or1100° C. A winding temperature was set at any of 450° C., 520° C., 570°C., 600° C., 630° C., 650° C., and 720° C.

After being pickled with hydrochloric acid, the hot-rolled steel plateswere finished to a thickness of 1.8 mm at various cold rolling ratios.Target cross-sectional hardness at a time after the cold rolling was 250HV. According to some of the Working Examples and some of theComparative Examples, a hot-rolled steel plate was annealed at 690° C.and then cold rolled.

Each of these samples of cold-rolled steel plates was subjected tomeasurement of heat conductivity, measurement of cross-sectionalhardness, and measurement of an average diameter of particles of aprecipitated carbide (Nb-based carbide (NbC), V-based carbide (VC orV₄C₃ and hereinafter called VC), and Ti-based carbide (TIC)), andmeasurement of the volume fraction of a hard structure in across-sectional structure.

Further, a test piece was extracted from each sample and was subjectedto a punching test, a pin-on-disk friction and wear test, and a heatspot resistance test.

For the measurement of heat conductivity, heat conductivity was measuredin a range from 100 to 200° C. using a laser flash method. A test piecewith measured heat conductivity of 50 W/m·K or more was evaluated asbeing good and is identified with o in Table 2.

For the measurement of cross-sectional hardness, a part of each samplewas cut out, buried in resin, and polished. Then, Vickers hardness wasmeasured at a thickness central part of a cross section. A measuringload was 50 gf.

For the measurement of an average diameter of particles of a carbide ina front layer, a part of each sample was cut and buried in resin so asto obtain an observation surface at one surface of a cold-rolled steelplate. Then, this part was polished parallel to the surface of thecold-rolled steel plate so as to obtain an observation surface in aposition at a depth from 50 to 150 μm in a thickness direction from thesurface of the cold-rolled steel plate. Then, this part was etched toproduce an extraction replica and a precipitate was observed. RegardingNb, V, and Ti, an observed precipitate resulting from addition of Nbalone was NbC, an observed precipitate resulting from addition of Valone was VC, and an observed precipitate resulting from addition of Tialone was TiC. Further, an observed precipitate resulting from combinedaddition was a carbide corresponding to any of NbC, VC, and TiC. Atransmission electron microscope (TEM) was used for the observation. Thesize of the precipitate was determined in terms of a circle with animage analyzer and the diameter of each precipitate was calculated. Animaging magnification ratio was 50,000 and ten fields of view wereobserved. A sum of the calculated particle diameters of the precipitateswas divided by the number of the precipitates to obtain an averageparticle diameter. Regarding the average particle diameter of theprecipitates of each sample measured in this way, an average particlediameter below 20 nm is identified with A, an average particle diameterfrom 20 to 100 nm is identified with B, and an average particle diameterexceeding 100 nm is identified with C in Table 2.

The area fraction (ratio) of a hard structure was measured by thefollowing procedure. A part of each sample was cut and buried in resinso as to obtain an observation surface at a plane including a rollingdirection and a thickness direction of a cold-rolled steel plate. Thispart was thereafter mirror finished through wet polishing and buffingand then etched with 5% of natal. Then, a structure was observed with ascanning electron microscope. A second phase structure means α phase ora structure different from the ferrite phase structure as a motherphase. The size of this second phase structure was observed in a visualsense under the microscope or in a photograph. Regarding the size of thesecond phase structure, a longitudinal diameter was used as arepresentative value.

For the punching test, a circular hole of a thickness of 1.8 mm and adiameter of 10 mm was punched out of each test piece using a 300 kNuniversal tester. For a punch die, SKD11 conforming to the JIS standardsmainly for cold work dies having a punch and a dice both adjusted to 60HRC was used. The test was conducted under conditions of a punchingspeed of 1.7 mm/sec. and a clearance of 5%. A punched item with 20 to 30punching shots was collected and was evaluated in terms of the amount ofsagging at a shear surface and a primary shear surface ratio. Morespecifically, each index was measured and an average was calculated in adirection in which a material steel plate was rolled and in a directionat a right angle to the rolling direction. A test piece with a primaryshear surface ratio of 50% or more and sagging below 0.2 mm wasevaluated as being good and is identified with o in Table 2. Referringto Table 2, a test piece with a primary shear surface ratio below 50% isidentified with ▴, a test piece with sagging of 0.2 mm or more isidentified with ▾, and a test piece subjected to the occurrence of asecondary shear surface is identified with x.

For the pin-on-disk friction and wear test, the wear test was conductedusing a pin-on-disk friction and wear tester while mission oil wasdropped. More specifically, a wear test test piece of 10 mm by 30 mm wasformed through a punching process (clearance of 5%) out of a cold-rolledsteel plate of a thickness of 1.8 mm. This wear test test piece wasfixed so as to form a test surface of 1.8 mm by 10 mm to contact a diskin the pin-on-disk friction and wear tester. A quenched and tempereditem of S45C having hardness of 450 HV was used as the disk. While thewear test test piece was pressed against the disk under a test load of100 N, the wear test was conducted under the conditions of a frictionspeed of 0.6 m/sec. and a friction distance of 800 m. A wear test testpiece with a wear height below 0.1 mm was evaluated as being good and isidentified with o in Table 2. In Table 2, a wear test test piece with awear height of 0.1 mm or more is identified with ▴ and a wear test testpiece subjected to serious adhesion is identified with x.

If a difference in a primary shear surface ratio is generated betweentest pieces resulting from a punching process, a larger primary shearsurface ratio makes a substantial contact surface pressure lower,showing a tendency to reduce a wear amount. Specifically, not only theantiwear performance of steel itself but also a primary shear surfaceratio becomes an important factor relating to a wear amount.

The heat spot resistance test (rapid heating and rapid cooling test) wasconducted through a method of heating a front layer part locally byapplying powerful laser light for a short period of time to a surface ofa test piece. Specifically, after the surface of the steel plate washeated with the laser light, the laser irradiation was stopped. Then,the self-cooling effect of the steel plate rapidly cooled the heatedpart to produce a characteristic altered layer (layer containingmartensite subjected to structure change) in a heat spot. Even in theabsence of the martensitic phase, temperature increase resulting fromthe laser irradiation may cause recrystallization of a material steelplate hardened by cold rolling to form coarse crystal grains, therebyreducing hardness in some cases. If the altered layer is formed in thisway resulting from either hardening or softening, a difference isgenerated between cross-sectional hardness inside the material steelplate and hardness at the laser-irradiated part. This allows evaluationof heat spot resistance by means of measurement of cross-sectionalhardness at a front layer part and internal cross-sectional hardness.

The following describes in detail how this heat spot resistance test wasconducted. As shown in FIG. 1, a test piece 1 of 25 mm by 25 mmextracted from each sample was fixed with a bolt not shown in thedrawings to a central part of a surface of a steel block 2 of 60 mm by60 mm and a thickness of 20 mm. Then, a laser beam was applied to acentral part of a surface of the test piece 1. Regarding conditions forthe irradiation, a CO₂ laser was applied with an effective output of1080 W, a beam shape was 6 mmφ in diameter, and an irradiation time was0.75 seconds.

As shown in FIGS. 2A to 2C, in the cross section of the test piece 1after the laser irradiation, Vickers hardness was measured at a laserirradiated measuring site 3 a in a position distanced by 100 μm from asurface of a laser-irradiated part 3 irradiated with the laser and at athickness central part 4 corresponding to a central part of a thicknessdirection. Then, a degree of hardening or softening of the front layer(laser irradiated measuring site 3 a) relative to internal hardness (atthe thickness central part 4) was evaluated. In Table 2, a test piece 1generating a difference of ±50 HV or less between the hardness at thefront layer and the internal hardness was evaluated as being good and isidentified with o, a test piece 1 generating a difference exceeding 50HV but below 100 HV is identified with Δ, a test piece 1 generating adifference of 100 HV or more is identified with ΔΔ, and a test piece 1generating a difference below −50 HV is identified with ▾.

The test pieces were evaluated comprehensively. A test piece evaluatedas being good in all the following evaluations was determined to havingpassed the tests and is identified with o in Table 2: the ratio of ahard structure, evaluation of the nature of a punching surface throughthe punching test, evaluation of antifriction and antiwear performancethrough the pin-on-disk friction and wear test, and evaluation of heatspot resistance through the heat spot resistance test.

Respective conditions for the tests and test results are shown in Table2.

TABLE 2 Average Particle Hardness Diameter of Rapid of Cross Carbide inRatio of Heat Heatings Com- SAE- Test Hot Rolling Average Cold SectionFixed Layer Hard Punching Fatigue Com- Rapid prehensive No. 2 No.Classification Condition Coding speed Annealing Rolling (HY) Part (μm)Structures Test Test prehensive Cooling Test Evaluation Test  1Comparative Example 1250° C. heating to 570° C. winding 33° C./sec. NoYes 254 A ∘ ∘ ▴ ∘ x x  2 Comparative Example 1250° C. heating to 570° C.winding 93° C./sec. No Yes 265 B x ∘x ∘ x ΔΔ x  3 Comparative Example1250° C. heating to 570° C. winding 28° C./sec. No Yes 256 B x x ▴ ∘ Δ x 5 Comparative Example 1250° C. heating to 570° C. winding 28° C./sec.No Yes 363 B x Δx ▴ ∘ ∘ x  6 Comparative Example 1250° C. heating to570° C. winding 30° C./sec. No Yes 258 B ∘ ∘ ▴ ∘ x x  7 ComparativeExample 1250° C. heating to 570° C. winding 29° C./sec. No Yes 377 C ∘∘x ∘ ∘ ∘ x  8 Comparative Example 1250° C. heating to 570° C. winding32° C./sec. No Yes 347 B ∘ ∘x ∘ ∘ ∘ x  9 Comparative Example 1250° C.heating to 600° C. winding 33° C./sec. No Yes 246 B ∘ Δ Δ ∘ Δ x x 10Comparative Example 1250° C. heating to 600° C. winding 39° C./sec. NoYes 254 B x Δ x x ΔΔ x x 11-a Working Example 1250° C. heating to 570°C. winding 33° C./sec. No Yes 262 B ∘ ∘ ∘ ∘ ∘ ∘ ∘ 11-b ComparativeExample 1250° C. heating to 570° C. winding 33° C./sec. No Yes 199 B ∘ xx ∘ ∘ x 11-c Comparative Example 1250° C. heating to 730° C. winding 30°C./sec. No Yes 265 A ∘ ∘ ∘ ∘ x x 11-d Comparative Example 1100° C.heating to 570° C. winding 32° C./sec. No Yes 262 A ∘ ∘ ▴ ∘ x x 12-aComparative Example 1250° C. heating to 570° C. winding 32° C./sec. NoYes 262 B x x x ∘ ∘ x 12-b Comparative Example 1250° C. heating to 570°C. winding 25° C./sec. 690° C.-20 h Yes 206 B x x ▴ ∘ ∘ x 12-cComparative Example 1250° C. heating to 650° C. winding 30° C./sec. NoNo 431 A x *Δ * ∘ x x x 13-a Working Example 1250° C. heating to 650° C.winding 27° C./sec. No Yes 252 B ∘ ∘ ∘ ∘ ∘ ∘ ∘ 13-b Working Example1250° C. heating to 570° C. winding 30° C./sec. 690° C.-26 h Yes 272 B ∘∘ ∘ ∘ ∘ ∘ 13-c Comparative Example 1250° C. heating to 450° C. winding32° C./sec. No Yes 261 B x ▴ ▴ ∘ ▾ x 14 Working Example 1250° C. heatingto 450° C. winding 30° C./sec. No Yes 261 B ∘ ∘ ∘ ∘ ∘ ∘ 15 WorkingExample 1250° C. heating to 450° C. winding 32° C./sec. No Yes 256 B ∘ ∘∘ ∘ ∘ ∘ 16 Working Example 1250° C. heating to 450° C. winding 29°C./sec. No Yes 248 B ∘ ∘ ∘ ∘ ∘ ∘ 17 Working Example 1250° C. heating to450° C. winding 32° C./sec. No Yes 259 B ∘ ∘ ∘ ∘ ∘ ∘ 18 Working Example1250° C. heating to 450° C. winding 27° C./sec. No Yes 255 B ∘ ∘ ∘ ∘ ∘ ∘19 Working Example 1250° C. heating to 650° C. winding 26° C./sec. NoYes 260 B ∘ ∘ ∘ ∘ ∘ ∘ 20 Comparative Example 1250° C. heating to 600° C.winding 25° C./sec. No Yes 247 B x x ▴ ∘ ∘ x 21 Working Example 1250° C.heating to 600° C. winding 28° C./sec. No Yes 266 B ∘ ∘ ∘ ∘ ∘ ∘ 22Working Example 1250° C. heating to 600° C. winding 33° C./sec. No Yes263 B ∘ ∘ ∘ ∘ ∘ ∘ 23 Working Example 1250° C. heating to 600° C. winding31° C./sec. No Yes 261 B ∘ ∘ ∘ ∘ ∘ ∘ 24 Working Example 1250° C. heatingto 600° C. winding 28° C./sec. No Yes 258 B ∘ ∘ ∘ ∘ ∘ ∘ 25 WorkingExample 1250° C. heating to 600° C. winding 32° C./sec. No Yes 246 B ∘ ∘∘ ∘ ∘ ∘ 26 Working Example 1250° C. heating to 630° C. winding 27°C./sec. No Yes 238 B ∘ ∘ ∘ ∘ ∘ ∘ 27 Working Example 1250° C. heating to630° C. winding 25° C./sec. No Yes 282 B ∘ ∘ ∘ ∘ ∘ ∘ ∘ 28 WorkingExample 1250° C. heating to 630° C. winding 32° C./sec. No Yes 234 B ∘ ∘∘ ∘ ∘ ∘ 29 Working Example 1250° C. heating to 630° C. winding 30°C./sec. No Yes 278 B ∘ ∘ ∘ ∘ ∘ ∘ 30 Working Example 1250° C. heating to630° C. winding 25° C./sec. No Yes 297 B ∘ ∘ ∘ ∘ ∘ ∘ 31 Working Example1250° C. heating to 600° C. winding 23° C./sec. No Yes 323 B ∘ ∘ ∘ ∘ ∘ ∘∘ 32 Working Example 1250° C. heating to 600° C. winding 24° C./sec. NoYes 292 B ∘ ∘ ∘ ∘ ∘ ∘ 33 Working Example 1250° C. heating to 600° C.winding 23° C./sec. No Yes 262 B ∘ ∘ ∘ ∘ ∘ ∘ 34 Comparative Example1250° C. heating to 520° C. winding 25° C./sec. 600° C.-20 h Yes 269 B xx ▴ ∘ ∘ x ∘ 35 Comparative Example 1250° C. heating to 520° C. winding35° C./sec. 690° C.-20 h Yes 258 B x x ▴ ∘ ∘ x 36 Comparative Example1250° C. heating to 600° C. winding 28° C./sec. No Yes 258 B ∘ x ▴ ∘ ∘ x37 Working Example 1250° C. heating to 600° C. winding 31° C./sec. NoYes 262 B ∘ ∘ ∘ ∘ ∘ ∘

As shown in Table 2, all Working Examples achieved favorable results interms of punching performance, antifriction and antiwear performance,and heat spot resistance.

Regarding test No. 1 corresponding to a Comparative Example, Nb and Vwere not added while a small quantity of Ti was added, so thatsubstantially no fine carbide was precipitated. This is considered to bea cause for reduction in antiwear performance, thereby softening asurface during the rapid heating and rapid cooling test.

Regarding test No. 2 corresponding to a Comparative Example, adding C toa content larger than 0.08% is considered to be a cause for increase ina hard structure, thereby degrading the nature of a punching surface.This large content of C is also considered be a cause for martensitictransformation occurring in a rapidly cooled part during the rapidheating and rapid cooling test, thereby hardening a surface to cause badheat spot resistance while reducing heat conductivity.

Regarding test No. 3 corresponding to a Comparative Example, adding C toa content larger than 0.08% is considered to be a cause for increase ina hard structure, thereby degrading the nature of a punching surface.Further, the Q value larger than 1 is considered be a cause formartensitic transformation occurring in a rapidly cooled part during therapid heating and rapid cooling test, thereby hardening a surface toreduce heat spot resistance.

Regarding test No. 5 corresponding to a Comparative Example, adding C toa content larger than 0.08% is considered to be a cause for increase ina hard structure. Further, adding Si to a content larger than 1.0% isconsidered to be a cause for reduction in processability, therebydegrading the nature of a punching surface.

Regarding test No. 6 corresponding to Comparative Example, adding C to acontent smaller than 0.03% is considered to be a cause for a smallquantity of a precipitated carbide or cementite, thereby reducingantiwear performance and softening a surface during the rapid heatingand rapid cooling test.

Regarding test No. 7 corresponding to a Comparative Example, adding Tito a content larger than 0.3% is considered to be a cause for hardeningof a material, thereby degrading the nature of a punching surface.

Regarding test No. 8 corresponding to a Comparative Example, the valuelarger than 0.3 of the formula (2) relating to the respective contentsof Ti and Nb is considered to be a cause for hardening of a material,thereby degrading the nature of a punching surface.

Regarding test No. 9 corresponding to a Comparative Example, adding Mnto a content larger than 0.8% and the Q value larger than 1 areconsidered to be causes for degradation of the nature of a punchingsurface, hardening of a surface during the rapid heating and rapidcooling test, and degradation of antiwear performance.

Regarding test No. 10 corresponding to a Comparative Example, adding Mnto a content larger than 0.8% and the Q value larger than 1 areconsidered to be causes for degradation of the nature of a punchingsurface and hardening of a surface during the rapid heating and rapidcooling test. Further, adding C to a content larger than 0.08% isconsidered to be a cause for reduction in heat conductivity, degradationof the nature of a punching surface, and reduction in antiwearperformance.

Regarding test No. 11-b corresponding to a Comparative Example,cross-sectional hardness lower than 200 HV is considered to be a causefor degradation of the nature of a punching surface (occurrence ofsagging) and reduction in antiwear performance.

Regarding test No. 11-c corresponding to a Comparative Example, awinding temperature higher than 700′C is considered to be a cause forreduction in the diameter of particles of a carbide precipitated in afront layer in response to progressing of surface decarburization,thereby softening a surface during the rapid heating and rapid coolingtest.

Regarding test No. 11-d corresponding to a Comparative Example, aheating temperature below 1200° C. during hot rolling is considered tobe a cause for insufficient dissolving of a carbide and suppression offine precipitation, thereby reducing a carbide precipitated in a frontlayer to reduce antiwear performance while softening a surface duringthe rapid heating and rapid cooling test.

Regarding test Nos. 12-a and 12-b corresponding to Comparative Examples,adding C to a content larger than 0.08% is considered to be a cause forincrease in a hard structure, thereby degrading the nature of a punchingsurface while softening a surface during the rapid heating and rapidcooling test.

Regarding test No. 12-c corresponding to a Comparative Example, awinding temperature below 550° C. is considered to be a cause forcoarsening of a carbide precipitated in a front layer, thereby reducingpunching performance to degrade the nature of a punching surface.

Regarding test No. 13-c corresponding to a Comparative Example, awinding temperature below 550° C. is considered to be a cause for a finecarbide precipitated in a front layer and increase in a hard structureto harden a material, thereby degrading the nature of a punching surfacewhile softening a surface during the rapid heating and rapid coolingtest.

Regarding test No. 20 corresponding to a Comparative Example, adding Cto a content larger than 0.08% is considered to be a cause for increasein a hard structure, thereby degrading the nature of a punching surfacewhile reducing antiwear performance.

Regarding test Nos. 34 and 35 corresponding to Comparative Examples,adding C to a content larger than 0.08% is considered to be a cause forincrease in a hard structure, thereby degrading the nature of a punchingsurface while reducing antiwear performance.

Regarding test No. 36 corresponding to a Comparative Example, adding Sto a content larger than 0.01% is considered to be a cause fordegradation of the nature of a punching surface and reduction inantiwear performance.

Regarding each of the test Nos. 11-a, 13-a, 27, and 31 corresponding toWorking Examples and test Nos. 9, 10, 12-c, and 34 corresponding toComparative Examples, a test piece was actually formed into the shape ofa separator plate and was subjected to a clutch performance test with anSAE-No. 2 tester as a clutch friction tester. Then, the presence orabsence of a heat spot was observed visually.

As a result, the occurrence of a heat spot was not observed in any ofthe Working Examples. In contrast, the occurrence of a heat spot wasobserved in every Comparative Example except test No. 34.

The invention claimed is:
 1. A cold-rolled steel plate having a chemicalcomposition containing, on the basis of percent by mass, C from 0.03 to0.08%, Si from 0 to 1.0%, Mn from 0.2 to 0.8%, P at 0.03% or less, S at0.01% or less, and Al at 0.05% or less so as to satisfy the followingformula (1) and at least one of Nb from 0.03 to 0.4%, V from 0.01 to0.3%, and Ti from 0.01 to 0.3% so as to satisfy the following formula(2), with a residue being formed of Fe and unavoidable impurities,wherein an average diameter of particles of a carbide as a precipitatecontaining any of Nb, V, and Ti is from 20 nm to 100 nm, a second phasestructure having a longitudinal diameter of 5 μm or more has an areafraction of 5% or less and above 0% in a cross-sectional structure, andthe cold-rolled steel plate has cross-sectional hardness from 200 to 350HV,5*C %−Si %+Mn %−1.5*Al %<1; and  formula (1)0.04<(Nb %/1.4)+(V %/1.1)+Ti %<0.3.  formula (2)
 2. The cold-rolledsteel plate according to claim 1, wherein the chemical compositioncontains, on the basis of percent by mass, at least one of Cr from 0.10to 2.0%, Ni from 0.05 to 0.5%, Mo from 0.05 to 0.5%, and B from 0.0002to 0.002% so as to satisfy a5*C %+Mn %+1.6*Cr %+0.8*Ni %−1.5*Al %<1.  formula (3)
 3. The cold-rolledsteel plate according to claim 1, wherein the average diameter ofparticles of the carbide containing any of Nb, V, and Ti is from 20 nmto 100 nm, the carbide being a precipitate in a front layer partextending at least 200 μm from a surface of the steel plate.
 4. A methodof manufacturing a cold-rolled steel plate, wherein a steel slab havingthe chemical composition as recited in claim 1 is smelted, the steelslab is heated to 1200° C. or more and hot rolled to form a hot-rolledsteel plate, the hot-rolled steel plate is wound from 550 to 700° C. toform a hot-rolled coil, and the hot-rolled coil is cold rolled or thehot-rolled coil is annealed and cold rolled, thereby obtainingcross-sectional hardness from 200 to 350 HV.
 5. The cold-rolled steelplate according to claim 2, wherein the average diameter of particles ofthe carbide containing any of Nb, V, and Ti is from 20 nm to 100 nm, thecarbide being a precipitate in a front layer part extending at least 200μm from a surface of the steel plate.
 6. A method of manufacturing acold-rolled steel plate, wherein a steel slab having the chemicalcomposition as recited in claim 2 is smelted, the steel slab is heatedto 1200° C. or more and hot rolled to form a hot-rolled steel plate, thehot-rolled steel plate is wound from 550 to 700° C. to form a hot-rolledcoil, and the hot-rolled coil is cold rolled or the hot-rolled coil isannealed and cold rolled, thereby obtaining cross-sectional hardnessfrom 200 to 350 HV.
 7. The cold-rolled steel plate according to claim 1,wherein the second phase structure having a longitudinal diameter of 5μm or more has an area fraction of greater than 0%.